Reactive adhesives based on polymerizable systems of the methacrylate adhesive type are normally marketed and applied in two-component form. It is only in use that the polymerizable component is mixed or otherwise suitably contacted with the hardener. The usual hardener systems contain peroxide-amine combinations as initiators for the radically initiated hardening reaction.
It is also known that suitable organoboron compounds act as reaction initiators under the effect of atmospheric oxygen in polymerizable systems containing double bonds. Thus, U.S. Pat. No. 4,381,386 describes two-component reactive adhesives in which non-inflammable organoboron compounds, particularly boron alkyl compounds, perform the initiator function. Two-component reactive adhesives which are based on boron alkyl compounds such as these as initiators, but which are applied in the form of so-called no-mix adhesives, are also described in European Patent Application No. 78,995. The disadvantage of all systems based on boron alkyl compounds as polymerization initiators lies in the inadequate stability of the boron alkyl compounds in storage on account of their high reactivity to oxygen. This also makes them difficult to handle in use.
Proposals to stabilize reactive boron compounds by mixing them or even by reacting them with oligomeric and/or polymeric components which are spreadable to solid at normal temperature and the use of polymerization initiators based on organoboron compounds stabilized in this way are the subjects of published European Patent Application Nos. 85,836; 84,805; 87,708 and 78,994. The handling of two-component reactive compositions, i.e. for example corresponding acrylatebased adhesives, is made very much easier in this way.
According to European Patent Application No. 51,796, however, it is also possible to formulate one-component curable (meth)acrylate adhesives using boron alkyls which react with atmospheric oxygen in a radical chain reaction. These aerobically curing (meth)acrylate adhesives are mixtures spreadable to fluid under normal conditions, particularly at room temperature, of
(a) suitable radically polymerizable monomers. PA1 (b) suitable polymers which influence the rheology of the uncured adhesive as required and provide for elasticity and flexibility in the cured adhesive film, PA1 (c) suitable boron alkyl as radical formers in the presence of air and, PA1 (d) stabilizers, inhibitors and accelerators. PA1 (a) at least one reactive component containing at least one polymerizable ethylenic C=C double bond; PA1 (b) at least one organoboron compound capable of initiating the polymerization of the ethylenically unsaturated compounds of component (a); and PA1 (c) optionally, a polymer compound having a melting or softening range of from 50.degree. to 350.degree. C. suitable as a hotmelt component; PA1 (d) anionic and/or radical polymerization inhibitors or stabilizers;
These multi-component mixtures have to be prepared and also stored in the complete absence of oxygen. When they come into contact with air on leaving the pack, the radical polymerization reaction (curing of the adhesive) is initiated. Despite favorable application properties and the advantage of a "one-package" formulation, the comparatively high cost of production in the complete absence of oxygen and, above all, the corresponding storage of one-component formulations of the type in question have hitherto been obstacles to their utilization in practice.
Another known class of adhesives are the storage-stable hotmelts which are widely used in practice. They are applied after melting of the polymer composition which is solid at room temperature, for example using the melt applicator guns known in the do-it-yourself field. The molten adhesive establishes the bond and fills the joint, hardening again on cooling. However, bonds established in this way are not heat-resistant.
Another known class of reactively hardening compositions, which are used not only as adhesives but also as cast resins, molding compounds and gap-filling compounds and the like, are the moisture-curing polyurethane systems which are generally used as one-component systems and which cure under the effect of atmospheric moisture. The desired three-dimensional crosslinking and, hence, the transition to the solvent-resistant and a no longer meltable state occurs during their application.